Membrane bioreactor (MBR) systems are becoming an increasingly popular solution for water and wastewater treatment. Although membrane systems for water treatment and purification have been in use for decades, the employment of MBR systems as a widespread solution for water and wastewater treatment has generally been disregarded in favor of more conventional biotreatment plants. One significant reason for such disregard is that MBR systems are often comparatively more expensive than conventional treatment systems. However, the higher purity of the product and the decreased footprint make the employment of MBR systems desirable.
MBR systems typically include one or more biological reactors, such as anaerobic, anoxic and aerobic reactors, followed by one or more membrane tanks with each tank containing one or more membrane modules. Water or wastewater is induced into the membrane modules by gravity feed or suction created by a pump. During the process, the membranes filter out contaminants and other solids and a permeate is produced.
One major drawback to membrane filtration processes is membranes tend to foul. As the membranes foul, the permeability of the membranes decrease, and the effectiveness of the whole process is reduced. It is generally understood that the rate of membrane fouling is increased roughly exponentially with an increase in the flux. Study of this phenomenon has lead to the theory of critical flux. Although critical flux is described in a number of ways, the general definition of critical flux is the flux below which permeability decline is considered negligible. Therefore, controlling the flux, preferably maintaining it at or below the critical flux, reduces the rate of permeability decline and provides sustainable operation of membrane systems.
Even if a membrane system is run at or below the critical flux rate, membrane fouling still occurs and methods of cleaning the membranes must be employed. In membrane systems such as MBRs, air scouring is often utilized to continually clean the membranes and help sustain permeation. Air scouring creates turbulence and shear force at the surface of the membrane to help reduce fouling and cake layer buildup. However, air scouring significantly increases operating costs and is not completely effective at maintaining adequate critical flux rates.
Other physico-mechanical and/or chemical membrane cleaning or treatment methods are used to remove fouling material and maintain membrane permeability. Most widely used physico-mechanical methods include backwashing, vibration, and air-scouring. These methods are energy-intensive and not applicable to all membrane types.
Chemical cleaning or treatment methods include pretreatment with coagulants and/or polymers, and treatment with antiscalants, biocides, and/or cleaning products such as NaOCl or citric acid. Mineral or organic acids, caustic soda, or sodium hypochlorite are also often used in chemical cleaning methods. However, frequent chemical cleaning is costly due to the loss in system operation time, decreased life expectancy of the membranes, and large consumption of cleaning chemicals.
Physical cleaning methods such as air scouring are most effective at removing gross solids from the membranes, the substances that cause fouling sometimes referred to as “temporary” or “reversible” fouling. Chemical cleaning methods are effective at removing more tenacious fouling substances, the substances that cause fouling sometimes referred to as “irreversible” or “permanent” fouling. However, chemical cleaning cannot remove all permanent or irreversible fouling substances and residual resistance of the membrane remains. This residual resistance or “irrecoverable” fouling is the fouling that builds up on the membrane over a number of years and ultimately limits the lifetime of the membrane.
Combinations of the mentioned methods are also commonly used, such as chemically enhanced backwashing, often as a daily cleaning measure. Weekly cleaning measures may include cleaning with higher chemical concentration, and less often regular cleaning may include even more intensive chemical cleaning with a significant negative effect on membrane lifespan.
The mechanisms of membrane fouling have been studied extensively. Fouling occurs over time and often in various stages depending upon flux rate and consistency, as well as the composition of the substance being passed through the membrane. The stages of fouling are sometimes described as initial fouling (or conditioning fouling), steady fouling, and transmembrane pressure (TMP) jump. Initial fouling is believed to be a result of colloid adsorption, small particulates blocking the membrane pores, and small flocs or extracellular polymeric substances (EPS) left from temporary attachment of biological aggregates to the membrane. The overall resistance change by this initial fouling often has only a negligible effect on flux and TMP once active filtration occurs. However, initial fouling is believed to play a bigger role in providing a favorable matrix for further or steady fouling. The steady fouling stage includes further pore blocking by particulate matter, but is also disadvantageous due to increased cake formation and biofilm growth on the membranes. This stage of fouling does not always occur homogeneously across the membrane, but steady fouling increases TMP and decreases permeability, resulting in a decrease in flux. The final stage of fouling is referred to as TMP jump where permeation lessens significantly in a relatively short period of time. There are a number of theories postulating the mechanisms that cause TMP jump. However, regardless of the mechanism, once TMP jump occurs, the membrane is so significantly fouled that it often is ineffective for use in the process.
Other process parameters can affect membrane flux. One example is the temperature that the process is run at. Generally, an increase in process temperature results in an increased flux rate. This flux improvement with higher temperature may be due to a decrease in permeate viscosity, and may decrease the rate of fouling. However, controlling the temperature of the water or wastewater treatment process is typically not feasible and would be cost prohibitive.
Solutions to reduce or prevent membrane fouling have targeted all types and stages of fouling. Particularly, targeting biofilm formation has been of recent interest. For example, Yeon et al., 2009, Environ. Sci. Technol. 43: 380-385 discuss targeting the quorum sensing (QS)-based membrane fouling mechanism of organisms that are involved in steady fouling.
U.S. Patent Application Publication No. 2008/0233093 discloses a small number of strains of the genus Bacillus that can reduce and/or prevent biofilm formation and/or planktonic proliferation when co-cultured with certain undesirable microorganisms.
Due to the critical need for effective water and wastewater treatment, solutions that decrease membrane fouling and/or increase critical flux rates in membrane applications including MBR systems are highly desirable.